The tuning bandwidth of a conventional single loop analog synthesizer often has a narrow tuning bandwidth (below 20% bandwidth), due to limitations in the tuning range of the varactor diode. More particularly applicable to low band and mid band ranges, if a high resonator Q is desired for good sideband noise performance, then a large and bulky resonating coil is usually required. The typical techniques used to diminish these problems involves the use of the divide down synthesis method or the mix down synthesis method.
Use of the divide down synthesis method generally reduces the size of the coil when a higher frequency voltage controlled oscillator (VCO) is used, but fails to provide an adequate tuning range due to the limits of the divider ratio. The conventional mix down method eliminates the problems with coil size and limits in tuning range, but adds the additional problem of inherent in-band spurious response signals due to Able-Baker mixing products. The Able-Baker spurious response signals cannot be filtered out due to their proximity to the desired signal over a wide frequency band. Therefore, a need for a mix down synthesizer method that reduces the in-band spurious response signal problems is required.